A Contra Costa County sheriff's deputy told two colleagues he was making "dirty DUI" arrests on behalf of a private investigator who was setting up men to damage their reputations in legal fights, an investigator said in court documents.

Deputy Stephen Tanabe, 47, told a reserve sheriff's deputy on patrol with him the night of Jan. 14 that a drunken-driving arrest the two were making outside a Danville bar was a "setup" and that the target needed to be "dirtied" for a future court date, a sheriff's investigator wrote in a search warrant affidavit.

Tanabe was working with a private investigator who had been hired by the target's wife for $5,000, the affidavit said. The deputy is suspected of receiving "financial benefits" for the arrest, the document said.

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Tanabe, an Alamo resident, was arrested Friday night on suspicion of drug and weapons crimes but has not been charged. He is free on $260,000 bail.

Illegal assault rifle

The weapons count involves an illegal assault rifle that Tanabe allegedly asked the reserve deputy, William Howard, to keep in his house. Tanabe was afraid his home would be searched after police arrested the private investigator, Christopher Butler, and a state drug agent, Norman Wielsch, on charges of stealing and selling drugs last month, the affidavit said.

The Chronicle first reported Sunday that investigators from the state Justice Department and Contra Costa County district attorney's office were reviewing two arrests that Tanabe made in January outside the Vine wine bar on Hartz Avenue in Danville to determine whether the deputy was working on Butler's behalf.

Contra Costa prosecutor Jun Fernandez said in court last week that Butler hired attractive women to make passes at male targets of his private-investigator firm and suggest they meet for drinks at a local bar. In other cases, Butler used decoys posing as journalists or documentary filmmakers who wanted to conduct lengthy interviews with their subjects over drinks, the prosecutor said.

In each case, Fernandez said, Butler would call police officer contacts and give a description of the target, the car he was driving and the moment he left the bar. After the man drove from the parking lot, the officer would fall in behind and arrest him.

Fernandez did not identify Tanabe in court as one of the officers working with Butler. But the deputy's alleged connection was spelled out in a search warrant affidavit by sheriff's Sgt. Detective Jason Vorhauer, which was filed Monday in Contra Costa County Superior Court.

Sobriety updates

The Jan. 14 arrest happened outside the Danville wine bar, according to the affidavit. The reserve deputy said Tanabe was on the phone that night with his "P.I. friend," later identified as Butler, who appeared to be "giving him updated information relating to (the target's) sobriety," Vorhauer wrote.

Butler apparently was waiting outside the bar and told Tanabe what kind of pickup the man was driving, Vorhauer wrote in the affidavit.

When the man pulled away from the bar, Tanabe arrested him. Howard later told the sheriff's investigator that he "felt uncomfortable with the arrest, but because of his inexperience did not question the incident," Vorhauer wrote.

A district attorney's investigator later said the man's wife had paid Butler about $5,000 to investigate her husband, Vorhauer wrote in the affidavit.

Two other cases

Tanabe allegedly made two other drunken-driving arrests to help Butler. Text messages between Tanabe and Butler tied the two to an arrest Jan. 9, and another deputy went to investigators with concerns about a Nov. 2 arrest, the affidavit said.

In that case, Deputy Tom Henderson said an off-duty Tanabe had called him to say a man "was drinking heavily" and would soon be leaving a Danville bar, Vorhauer wrote.

Tanabe told the deputy that the man "was cheating on his wife and (Butler and Tanabe) wanted to 'dirty him up' for a future court case," the sheriff's investigator wrote. Henderson arrested the man and booked him on suspicion of drunken driving, the affidavit said.

Tanabe, who has been put on paid leave from the sheriff's department, has not responded to phone messages. It is not known whether he has an attorney.